Pass a "struct ifnet *" instead of a "struct arpcom *" to arpresolve().

Pass a "struct ifnet *" instead of a "struct arpcom *" to arpresolve().

Most of the ARP layer already take an ifp pointer and this makes clear
wich chunks of code are messing with ac_enaddr.

Note that our Ethernet code assume that these pointer are interchangeable
since the first element of the "struct arpcom" is a "struct ifnet".